EP1883351B1 - Method for predicting a blood glucose level of a person - Google Patents

Description

Field of the Invention
• The current invention relates to methods for predicting a blood glucose level of a person using a near-infrared spectral scan of a body part of the person.
Description of Prior Art
• People affected with diabetes must frequently monitor their blood glucose level. The traditional method of checking blood glucose level involves a finger prick to draw a drop of blood that is tested in an analytical device. It is often difficult, particularly for children and elderly people, to perform this test especially if it is needed several times a day.
• There has been considerable research into non-invasive methods of predicting the blood glucose level of a person affected with diabetes. A popular method involves using a near-infrared (NIR) spectral scan of a body part of the person. See, for example, the method and apparatus for rapid non-invasive determination of blood composition parameters described in US Patent 5,974,337 (Kaffka et al ). When NIR light is radiated through the skin and into the blood vessels glucose molecules in the blood absorb some of the NIR light energy. The corresponding NIR absorbance can be used to predict the glucose level of the blood. The major problem with this method is accurately establishing an evaluation model for predicting the blood glucose level from the NIR spectral scan results. Various methods of establishing evaluation models are given in US patent 6,675,030 to Ciurczak et al and its various references.
• A problem with most of the disclosed methods is that they are usually specific to the person being tested and the evaluation models are dynamic and often require recalibration.
Summary of the Invention
• It is an object of the present invention to provide a method for predicting a blood glucose level of a person using a near-infrared spectral scan that ameliorates the above mentioned problem or at least provides the public with a useful alternative.
• According to a first aspect of the invention there is provided a method for predicting blood glucose level of a person using a near-infrared spectral scan of a body part of the person, comprising:
• performing on a person a near-infrared spectral scan of a body part at a first group of wavelengths and at a second group of wavelengths,
• determining a first group of near-infrared absorbance values for the first group of wavelengths and a second group of near-infrared absorbance values for the second group of wavelengths,
• determining a first sum for the first group of near-infrared absorbance values and a second sum for the second group of near-infrared absorbance values, and
• calculating a blood glucose level for the person using the first and second sums;
• wherein the first and second sums are determined using an equation of the form: $$\mathrm{d}=\mathrm{A}•\mathrm{w}\mathrm{1}+\mathrm{B}•\mathrm{w}\mathrm{2}+\mathrm{C}•\mathrm{w}\mathrm{3}$$
  
  
  where d is the first or the second sum; w1, w2 and w3 are near-infrared absorbance values and A, B and C are constants; and wherein the blood glucose level is calculated using an equation of the form c = D + (E x (d1/d2))where c is the blood glucose level, d1 and d2 are the first and second reference values respectively and D and E are empirically determined constants.
• The method for predicting a blood glucose level of a person using a near-infrared spectral scan of a body part of the person further comprises providing an evaluation model based on a population of test subjects, the evaluation model relating near-infrared absorbance of blood vessels of a body part of the test subjects at first and second groups of wavelengths to a glucose level of the blood vessels.
• Preferably, the first group of wavelengths comprises first, second and third wavelengths and the second group of wavelengths comprises fourth, fifth and sixth wavelengths; wherein the first, second and third wavelengths are within a range from 750nm to 1125nm and the fourth, fifth and sixth wavelengths are within a range from 905nm to 1701mm.
• Preferably the first group of wavelengths comprises first, second and third wavelengths and the second group of wavelengths comprises fourth, fifth and sixth wavelengths, the first, second and third wavelengths are within a range from 750nm to 1125nm and the fourth, fifth and sixth wavelengths are within a range from 905nm to 1701mm; wherein the first through sixth wavelengths are determined using a recursive method based on a population of test subjects.
• Preferably, the constants D and E are determined using a linear regression based on a population of test subjects.
• According to a second aspect of the invention there is providedan apparatus for predicting blood glucose level of a person using a near-infrared (NTR) spectral scan of a finger of the person, comprising:
• a NIR light source and sensor for performing on a person a near-infrared spectral scan of a finger at a first group of wavelengths and at a second group of wavelengths; and
• an analyser for determining a first group of near-infrared absorbance values for the first group of wavelengths and a second group of near-infrared absorbance values for the second group of wavelengths, determining a first sum for the first group of near- infrared absorbance values and a second sum for the second group of near-infrared absorbance values, and calculating a blood glucose level for the person using the first and second sums;
• wherein the first and second sums are determined using an equation of the form: $$\mathrm{d}=\mathrm{A}•\mathrm{w}\mathrm{1}+\mathrm{B}•\mathrm{w}\mathrm{2}+\mathrm{C}•\mathrm{w}\mathrm{3}$$
  
  
  where d is the first or the second sum; w1, w2 and w3 are near-infrared absorbance values and A, B and C are constants; and wherein the analyser calculates the blood glucose level using an equation of the form: c = D + (E x (d1/d2))where c is the blood glucose level, d1 and d2 are the first and second reference values respectively and D and E are empirically determined constants.
• Further aspects of the invention will become apparent from the following drawings and description.
Brief Description of the Drawings
• Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:
• Figure 1 is a schematic illustration of apparatus for obtaining a near-infrared spectral scan of a body part of a person,
• Figure 2 illustrates correlation (or least square) coefficients of near-infrared absorbance of blood vessels of a body part at wavelengths of 940nm, 1310nm and 1550mn to a glucose level of the blood vessels, and
• Figure 3 is a flow chart illustrating a recursive method used to determine optimum spectral wavelength values.
Description of the Preferred Embodiments
• In figure 1 there is depicted apparatus for performing a near-infrared (NIR) spectral scan of a body part of a person. The apparatus comprises a NIR light source and sensor for obtaining an NIR scan of a finger. The NIR scan signal from the sensor is processed by an analyser and a prediction of the person's blood glucose level output on a display.
• To predict the blood glucose level of the person using the NIR spectral scan an evaluation model was found based on a population of test subjects. The evaluation model relates NIR absorbance of blood vessels at a first and a second group of wavelengths to a glucose level of the blood vessels. The graph in figure 2 shows the relationship between the NIR absorbance of blood of a body part of the test subjects and the mean value of laboratory tested glucose levels of the test subjects at three sample wavelengths of 940nm, 1310nm and 1550nm. The reference glucose levels are obtained using proven standard laboratory tests on blood samples from the test population. The linear relationship is found using a least squares method. The standard deviation on the mean value of the tested glucose levels for the test population was 10% to 20%.
• The evaluation model is based on NIR absorbance scan of the blood vessels at six wavelengths i-r1, i, i+r2; k-s1, k and k+s2: where i, k are primary wavelengths and r1, r2, s1 and s2 are empirical values obtain from the test population. The NIR absorbance data is obtained from a NIR spectral scan using the apparatus of Figure 1.
• For the evaluation model the six wavelengths are divided into a first group (i) of three wavelengths comprising i, i-r1 and i+r2; and a second group (k) of three wavelengths comprising k, k-s1 and k+s2. In the preferred embodiment all six wavelengths are within the range 750nm to 1700nm. However, in other embodiments the first group of wavelengths i, i-r1 and i+r2 may be within a range from 750nm to 1125nm and the second group of wavelengths k, k-s1 and k+s2 may be within a range from 905nm to 1701nm.
• The second sums (reference values) are determined for each group of wavelengths using the equation: $$d=\mathrm{A}\cdot w1+\mathrm{B}\cdot w2+\mathrm{C}\cdot w3$$

  

  where d is the second sum; w1, w2 and w3 are NIR absorbance values at three particular wavelengths in the respective groups of wavelengths i-r1, i, i+r2 or k-s1, k k+s2 and A, B and C are empirically determined constants. The preferred values of constants A, B and C are 1, -2 and 1 respectively. Substituting for the six wavelengths the two equations become: $$\mathrm{di}=\mathit{w}\left(\mathit{i}-\mathit{r}\mathit{1}\right)-2\mathit{w}\left(\mathit{i}\right)+\mathit{w}\left(\mathit{i}-\mathit{r}\mathit{2}\right)$$

  

  and $$\mathrm{dk}=\mathit{w}\left(\mathit{k}-\mathit{s}\mathit{1}\right)-2\mathit{w}\left(\mathit{k}\right)+\mathit{w}\left(\mathit{k}-\mathit{s}\mathit{2}\right)$$

  
• The ratio of the second sums di/dk is used to evaluate the glucose level of the person according to the equation: $$C=\mathrm{D}+\left(\mathrm{E\; x}\left(\mathit{di}/\mathit{dk}\right)\right)$$

  

  where, c is the predicted blood glucose level, and D and E are calibration coefficients for individual hardware obtained from a linear regression using the ratios of second sums and the reference glucose levels obtained from the population of test subjects.
• The optimum wavelengths in each group are determined using a recursive method. Figure 3 illustrates a suitable recursive method used to evaluate every possible ratio of second sum obtained from every possible combination of wavelength parameters, i, k, r1, r2, s1 and s2, from the NIR spectral scan. During each recursion, the current ratio of second sum is substituted into the evaluation equation. The best values for the constants D and E are determined by linear regression with cross-validation during each recursion. The evaluated values for D and E along with the current ratio of second sum produce an interim prediction model Ci = D + (E x (di/dk)). The fitness of the interim prediction model is evaluated. If the interim prediction model in the current recursion generates a better fitness than all the previous prediction models, the previously preserved prediction model is discarded and replaced with the current interim prediction model. Otherwise, the current prediction model is discarded and the previously preserved prediction model is not altered. The recursion repeats until all possible combinations of wavelength parameters, thus all possible ratios of the second sums are evaluated. When the recursion is finished the finally preserved prediction model which gives the best-fit result is selected as the final prediction model. The preferred ranges of constants D and E are between +/-30 and between +/-50, respectively.
• The optimum wavelength varies for different fingers within the range of 750nm to 1701nm. Preferred values for r1, r2, s1, s2 and the wavelengths for each finger are given in the following table.

  	r1	r2	I	i-r1	i+r2	S1	S2	k	k-s1	k+s2
  Thumb	7	7	977	970	984	7	7	1, 032	1,025	1,039
  Index finger	6	6	1,056	1,050	1062	6	6	1,511	1,505	1,517
  Middle finger	15	15	939	924	954	15	15	1,139	1,124	1,154
  Ring finger	4	4	1,409	1,405	1413	4	4	1,024	1,028	1,028
  Last finger	6	6	1,031	1,025	1037	6	6	1,511	1,505	1,517

• Where in the foregoing description reference has been made to integers or elements having known equivalents then such are included as if individually set forth herein.
• Embodiments of the invention have been described, however it is understood that variations, improvement or modifications can take place without departure from the scope of the appended claims.

Claims (8)

1. A method for predicting blood glucose level of a person using a near-infrared spectral scan of a body part of the person, comprising:

   performing on a person a near-infrared spectral scan of a body part at a first group of wavelengths and at a second group of wavelengths,

   and characterised in further comprising:

   determining a first group of near-infrared absorbance values for the first group of wavelengths and a second group of near-infrared absorbance values for the second group of wavelengths,

   determining a first sum for the first group of near-infrared absorbance values and a second sum for the second group of near-infrared absorbance values, and

   calculating a blood glucose level for the person using an equation of the form: $$\mathrm{d}=\mathrm{A}\cdot \mathrm{w}\mathrm{1}+\mathrm{B}\cdot \mathrm{w}\mathrm{2}+\mathrm{C}\cdot \mathrm{w}\mathrm{3}$$

   

   
   where d is the first or second sum; w1, w2 and w3 are near-infrared absorbance values and A, B and C are constants; and

   wherein the blood glucose level is calculated using an equation of the form: $$\mathrm{c}=\mathrm{D}+\left(\mathrm{E\; x}\left(\mathrm{d}\mathrm{1}/\mathrm{d}\mathrm{2}\right)\right)$$

   

   
   where c is the blood glucose level, d1 and d2 are the first and second sums respectively and D and E are empirically determined constants.
2. The method of claim 1 wherein the first group of wavelengths comprises first, second and third wavelengths and the second group of wavelengths comprises fourth, fifth and sixth wavelengths, wherein the first, second and third wavelengths are within a range from 750nm to 1125nm and the fourth, fifth and sixth wavelengths are within a range from 905nm to 1701 nm.
3. The method of claim 1 wherein the first group of wavelengths comprises first, second and third wavelengths and the second group of wavelengths comprises fourth, fifth and sixth wavelengths, wherein the first, second and third wavelengths are within a range from 750nm to 1125nm and the fourth, fifth and sixth wavelengths are within a range from 905nm to 1701nm; and wherein the first through sixth wavelengths are determined using a recursive method based on a population of test subjects.
4. The method of claim 1 wherein the constants D and E are determined using a linear regression based on a population of test subjects.
5. The method of any preceding claim further comprising:

   providing an evaluation model based on a population of test subjects, the evaluation model relating near-infrared absorbance of blood vessels of a body part of the test subjects at first and second groups of wavelengths to a glucose level of the
   / blood vessels.
6. An apparatus for predicting blood glucose level of a person using a near-infrared (NIR) spectral scan of a finger of the person, comprising:

   a NIR light source and sensor for performing on a person a near-infrared spectral scan of a finger at a first group of wavelengths and at a second group of wavelengths; and

   characterised in further comprising:

   an analyser for determining a first group of near-infrared absorbance values for the first group of wavelengths and a second group of near-infrared absorbance values for the second group of wavelengths, determining a first sum for the first group of near-infrared absorbance values and a second sum for the second group of near-infrared absorbance values, and calculating a blood glucose level for the person using the first and second sums;

   wherein the first and second sums are determined using an equation of the form: $$\mathrm{d}=\mathrm{A}\cdot \mathrm{w}\mathrm{1}+\mathrm{B}\cdot \mathrm{w}\mathrm{2}+\mathrm{C}\cdot \mathrm{w}\mathrm{3}$$

   

   
   where d is the first or second sum; w1, w2 and w3 are the near-infrared absorbance values and A, B and C are constants; and

   wherein the analyser calculates the blood glucose level using an equation of the form: $$\mathrm{c}=\mathrm{D}+\left(\mathrm{E\; x}\left(\mathrm{d}\mathrm{1}/\mathrm{d}\mathrm{2}\right)\right)$$

   

   
   where c is the blood glucose level, d1 and d2 are the first and second sums respectively and D and E are empirically determined constants.
7. The apparatus of claim 6 wherein the first group of wavelengths comprises first, second and third wavelengths and the second group of wavelengths comprises fourth, fifth and sixth wavelengths; the first, second and third wavelengths within a range from 750nm to 1125nm and the fourth, fifth and sixth, wavelengths within a range from 905nm to 1701nm.
8. The apparatus of claim 6 wherein the analyser determines the wavelengths using a recursive method based on a population of test subjects.

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